Latest environments and situations in broadcasting, which in itself has a long history in television and radio even as a digitalised one, have clearly created a need for evaluating the broadcast technology in a situation where it was not originally designed. For example, digital broadcast system such as DVB system has been evaluated in situations for which it was not originally designed, like mobile reception. Moreover perhaps in some other fields also, Orthogonal Frequency Division Multiplex (OFDM) radio technique, which is used for example in DVB-T, is similarly facing the risen mobility challenge.
Also some new usage applications for broadcasting technology, for example like IP-Data Casting (IPDC), have different usage scenarios and, hence, different requirements and challenges.
These have for their part created further requirements such as power consumption considerations. One solution for this has been so-called time slicing technology. In the examples of the broadcasting or OFDM having some power consumption considerations, for example portable IPDC like usage, the start-up time should be very fast because of the power saving aspects such as transmission and reception based on bursts. The time slicing is used to save power, and the synchronization into bursts have to be fast.
Current approaches utilises coarse symbol timing, which is based on guard interval correlation. An example of the guard interval correlation is shown in FIG. 1, where Nu denotes a symbol (or sometimes referred to as useful symbol interval), preferably used in OFDM signal. One OFDM symbol can contain N samples. Ng denotes guard interval part length. The current solutions uses also fine timing (FT), which is based on estimating the position of Channel Impulse Response (CIR). However, the coarse timing accuracy is not good enough so that the FT can always find the best possible Fast Fourier Transform (FFT) window placement.
As one approach so-called fallback procedure is used to cope with the inaccuracy of the coarse timing. A basic assumption is that coarse timing is detecting the first peak of the CIR (i.e. the beginning of the guard interval). However, for example the strong pre-echoes mixes this assumption, and the FFT-window placement is erroneous. The FT can handle the errors up to ½*(⅓*Nu-Ng), where similarly Nu is symbol part length and Ng is guard interval part length. In samples, the accuracy is as shown in Table 1.
TABLE 1Required coarse timing accuracy in samples.Maximum errors with different modes(guard interval size in brackets)Guardinterval2k4k8k 1/32309 (64) 618 (128)1237 (256) 1/16277 (128)554 (256)1109 (512)⅛ 216 (256)426 (512) 853 (1024)¼  85 (512) 170 (1024) 841 (2048)
Unfortunately, the required accuracy is higher the longer the guard interval is. Because of this perhaps, some further adjustment is needed, with two longest guard intervals. This fallback procedure is detecting the time synchronization failure by using Forward Error Correction (FEC) (BER/RS-lock) failure detection. Therefore, if the coarse timing, Transmission Parameter Signalling (TPS) and frequency are in lock and FEC fails, the Signal to Noise Ratio (SNR) is too low or fine timing acquisition has failed. To find out is the problem with the fine synchronization, pre-FFT position of the guard interval has to be shifted towards past, and new acquisition has to be started. The amount of shift depend perhaps how much error can be accepted. The recommended value can be 1.7×½*(⅓*Nu-Ng). This would require four fall back loops until the range of guard interval has been tried (the very worst case could be that the coarse timing is detecting the last peak of the channel impulse response). This is illustrated in the example of FIG. 2. FT-window (200) according to the coarse timing is shown in FIG. 2. Four trial positions (201) are also depicted. The uppermost can be seen the first trial position. The next trial position is shown below the first, and in the example the trial position is moved to the left. Also a search window (202) is depicted, which size equals to the guard interval length.
Because the fine timing (FT) is using time interpolated scattered pilots, four fallback loops require quite much time. With the current 8-tap time interpolation this will be 4×32 symbols, and the required time with different modes of operation of DVB-T transmission will be:    8 k: ˜140 ms (4-tap 70 ms)    4 k: ˜70 ms (4-tap 35 ms)    2 k: ˜35 ms (4-tap 18 ms)
One proposed improvement is to use linear (i.e. 2-tap) time interpolation during acquisition phase. With the linear interpolation, 4×4 symbols is required which will be    8 k: ˜20 ms    4 k: ˜10 ms    2 k: ˜5 ms
However, when using the linear interpolation in acquisition, yet some problems rises.
For example, the FEC failure detection may be unreliable because of insufficient time interpolation. In the presence of perhaps high Doppler, FEC failure may happen, although the FFT-window position may be correct. This will cause fine synchronization to try all the trial position, and because FEC failure is always there, the conclusion will be that the signal is too weak although with 8-tap/4-tap interpolation FEC failure wouldn't happen.
For another example, the 2-tap (i.e. linear) interpolation may cause so-called ghost peaks into the channel's impulse response. The ghost peaks (i.e. 302), shown in the example of FIG. 3, are perhaps caused by the high Doppler and insufficient time interpolation (time interpolated pilots will not match into frequency). These ghost peaks will obscure the search of guard interval position, as illustrated in the example with 4 k system in FIG. 3. A diagram (300) depicts IFFT with 4-tap time interpolation. The interference caused by the ghost peak (real peak/ghost peak) is 30 dB in the diagram (300). A diagram (301) depicts IFFT with 2-tap time interpolation. The interference caused by the ghost peak (Real peak/Ghost peak) can be 17 dB in the diagram (301). In the example of FIG. 3, Inverse FFF (IFFT) with 4 k mode and Doppler 120 Hz is applied.
In view of various limitation of broadcasting systems or multi-carrier radio technique, it would be desirable to avoid or mitigate these and other problems associated with prior art. Thus, there is a need for a faster symbol-timing operation in multi-carrier systems.